Field
The present disclosure generally relates to a technique for locking an optical receiver to its received wavelength. More specifically, the present disclosure relates to a technique for locking an optical receiver that is insensitive to the polarization of an optical signal.
Related Art
Optical interconnects or links based on silicon photonics have the potential to alleviate inter-chip communication bottlenecks in high-performance computing systems that include multiple processor chips and memory chips. This is because, relative to electrical interconnects, optical interconnects offer significantly improved: bandwidth, density, power consumption, size, latency, and range. As a consequence, researchers are investigating optical interconnects based on wavelength division multiplexing (WDM) for use in computing systems.
In many computing systems, relatively long interconnects are often needed, such as the interconnects between: processors, processors and memory, processing nodes, and racks. Typically, optical fiber is used for these long interconnects. FIG. 1 provides a block diagram of an existing optical system, with an optical fiber coupling a transmitter chip and a receiver chip. In the transmitter chip, WDM wavelength channels are modulated and multiplexed into an optical waveguide. Then, an optical signal that includes the multiplexed WDM wavelength channels is coupled into a transport optical fiber via an optical waveguide-to-optical fiber coupler. Moreover, at the receiver chip, the WDM optical signals are coupled back to a silicon optical waveguide on the receiver chip. Next, a wavelength de-multiplexer separates the wavelength channels into different receiver channels.
Because of the high contrast in the index of refraction and wavelength-scale dimensions, silicon optical waveguides on thin silicon-on-insulator are inherently polarization-dependent. While it is possible to create silicon-on-insulator (SOI) optical waveguides that guide both polarizations with low loss (such as ˜0.1 dB/cm) for sub-micron optical waveguides, typically, only the transverse-electric (TE) mode is propagated in the optical waveguide with low loss, while the transverse-magnetic (TM) mode is highly attenuated. Additionally, there is usually birefringence associated with the two different polarizations, which can cause the orthogonal polarizations to experience different indexes of refraction. Moreover, because most of the viable SOI modulators use sub-micron optical waveguides (e.g., a Mach-Zehnder modulator and resonant ring modulators), such silicon photonic circuits usually only provide functionality for a single polarization. Furthermore, because the polarization of light is an unknown and changing quantity in most optical fibers, this can pose a serious problem for silicon photonic WDM optical interconnects over optical fibers, especially at the receiver chip, which must deal with a varying and unknown state-of-polarization of the incoming optical signal.
As shown in FIG. 1, with an optical waveguide-to-optical fiber coupler, silicon-on-insulator optical waveguides, and a wavelength de-multiplexer supporting single polarization, the magnitude of the received optical signal will vary depending upon the polarization state of light in the optical fiber. The polarization dependence results in intensity noise that can degrade the performance, and thus the reliability of such a silicon-photonic WDM optical interconnect over an optical fiber.
One technique for addressing this problem is to use a polarization-maintaining (PM) optical fiber to control the polarization of light transmission to the receiver chip. However, this approach is often expensive and difficult to implement. In particular, PM optical fibers are usually much more expensive and far less readily available than regular single-mode optical fibers (SMFs). Furthermore, the PM optical fiber may need to be keyed at every coupling point to maintain its radial alignment.
Alternatively, the problem can be addressed by using a multimode optical fiber as a natural polarization scrambler. However, while this approach reduces the polarization sensitivity of the receiver chip that supports single polarization, it also introduces significant optical loss when coupling multimode WDM optical signals into silicon-on-insulator optical waveguides and can severely limit the range.
Hence, what is needed is an optical receiver that does not suffer from the above-described problems.